The invention relates to effecting video transitions between video streams with a border between the two partial images being displayed.
In editing of video in the creation of a video program, it is often desirable to provide a gradual transition from one video stream to another video stream. For example, in a wipe transition, a line may move across the screen from left to right, with the old scene gradually disappearing at the right while more and more of the new scene appears at the left. In a so-called xe2x80x9ciris wipe,xe2x80x9d the boundary is an expanding circle, and one video stream appears in an expanding circle, and the other is outside of the circle. A solid color could also be used as one stream to provide an image that gradually appears or disappears across the screen or an image that gradually expands or contracts.
Instead of directly transitioning from one video stream to another, there can be a border between the two which might move from one side of the screen to the other. FIG. 1 shows a wipe without a border going left to right across a screen; there is a transition from one video stream directly into the other. FIG. 2 shows the wipe with a border of a uniform color, with one transition from one video stream to the border, and another transition from the border to the other video stream.
Referring to FIG. 3, there is shown video combiner 10, which is a complex circuitry with arithmetic functions that can be used to combine video streams into a combined stream, and thus can be used for a transition from one video to another. Video combiner 10 has a first video input 12 for a first video stream, a second video input 14 for a second video stream, a key input 16 to receive key values indicating how the two inputs are to be combined, and a video output 18 for the output video that is some combination of the two inputs. The video streams include frames (each frame providing an image for a screen) made up of horizontal lines, each of which includes individual xe2x80x9cpixelsxe2x80x9d (picture elements) across the line. Each pixel value includes a number of bit values that describe the color and intensity at that particular pixel at a particular time. New frames are provided at the video rate, e.g., 30 frames per second, and the video streams include sequential digital data describing sequential pixel values for lines of video data in a frame. The pixels for the first and second video streams that are input to combiner 10 are synchronized, as are the associated key values. The key value for a given pixel indicates whether the output for that pixel is the input from one stream or the other stream or a combination of the two. The video combiner can be used to provide a gradual transition from one stream to the other, with an increasing percentage of the new stream and decreasing amounts of the old stream over a number of adjacent pixels. This is known as softness.
While a unique key value could be stored and accessed for each pixel in each frame, to reduce bandwidth, a single table of gamma values (also referred to as xe2x80x9cgradient valuesxe2x80x9d herein) can be used. For example, a single table with one value for each pixel can be used to define the wipe over the sequence of frames. In the table, the gamma value essentially indicates the time at which the transition appears at that pixel. FIG. 4 is a simplified diagram of a table 20 of gamma values for the wipe shown in FIG. 1. (A real table for NTSC would have 720 horizontal entries and 480 vertical entries.) As the time goes from frames 1 to 2 to 3 etc, a xe2x80x9cthresholdxe2x80x9d value similarly goes from 1 to 2 to 3 etc. The threshold is compared to the value in the table to determine the key value (e.g., a 0 key value meaning all of video stream A, a 100 key value meaning all of video stream B, and a 50 key value meaning equal amounts of A and B) that is fed to the video combiner. The key values that are generated cause the video output to switch over from one video input to the other. As the threshold increases (as the frame numbers increase), the boundary of the transition provided by use of the FIG. 4 table moves to the right. The following algorithm employs a direct comparison that provides an abrupt transition from the first video stream to the second.
If (gradient less than threshold) then all video A, else all video B.
As noted above, softness can be added to the key values generated so that the change from one video to the other is not abrupt but instead is gradual (with decreasing amounts of video B and increasing amounts of video A) in a discrete number of adjacent pixels as the transition passes a pixel location. The key value to the video combiner determines the percentage of video A or video B in the output video.
Transitions with borders, as in FIG. 2, have been provided in the past by using two video combiners 22, 24, a first video combiner 22 for combining video streams A and B and a second combiner 24 for combining the border, as shown in FIG. 5. In this apparatus, two video combiners, which are complex circuits, must be used. Also, two different gradient key streams must be provided, one to control video combiner 22 and one to control video combiner 24.
The invention features, in general, combining first and second video streams and a border color into a combined output video stream using a multiplexer and a video combiner. The multiplexer receives the first and second video streams as input and outputs a selected video stream in response to a control signal. The video combiner receives the border color at a first input and the selected video stream from the multiplexer at a second input and outputs a combined output based upon a key value received at a key input. This results in efficient, cost effective use of circuitry when combined multiple video streams with a border color.
Preferred embodiments of the invention may include one or more of the following features. In preferred embodiments a control signal RAM is connected to output the control signal to the multiplexer, and a key RAM is connected to output the key value to the video combiner. The control signal RAM and the key RAM are connected to be addressed by a sequence of gradient values from a gradient table. The key values stored in the key memory can gradually change over adjacent gradient values to desirably provide a gradual transition between a border color and a video stream input to the video combiner. The values in the control signal RAM and key RAM can be updated between frames. Alternatively, a subtracter or adder can be used to subtract an adjustment value that changes between frames from the values in a gradient value stream.
Embodiments of the invention may include one or more of the following advantages. Borders can be achieved in video wipes without the need for a second video combiner, which is replaced by a simple multiplexer, and without the need for a second key stream, because the same key stream can be used to control the video combiner and the multiplexer. In addition, the system desirably provides adjustable, accurate softness at the border/video transitions.
Other features and advantages of the invention will be apparent from the following description of a particular embodiment thereof and from the claims.